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Strontium and neodymium isotopes in hot springs on the East Pacific Rise and Guaymas Basin

Piepgras, D. J. and Wasserburg, G. J. (1985) Strontium and neodymium isotopes in hot springs on the East Pacific Rise and Guaymas Basin. Earth and Planetary Science Letters, 72 (4). pp. 341-356. ISSN 0012-821X. http://resolver.caltech.edu/CaltechAUTHORS:20131023-103534320

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Abstract

We have determined the concentrations and isotopic compositions of Sr and Nd in hydrothermal fluids from 21°N, East Pacific Rise and Guaymas Basin, Gulf of California. The purest solutions analyzed from 21°N exhibit a small range in Sr concentration between individual vents from 5.8 to 8.7 ppm, close to normal seawater Sr concentrations. They exhibit a small range in ^(87)Sr/^(86)Sr from ε_(Sr)(0) = −13.4 to −17.7, corresponding to ε_(Sr)(0) ≈ −18 ± 2 in the pure hydrothermal end-member. These results indicate extensive but not complete isotopic exchange with Sr in the depleted oceanic crust (ε _(Sr)(0) = −31.8) and suggest that Sr concentrations in these solutions are buffered. In contrast, the concentration and isotopic composition of Nd in solutions show large variations between vents. The concentration of Nd ranges from 20 to 336 pg/g (6–100 times seawater Nd concentrations). The isotopic composition ranges from ε_(Nd)(0) = −3.6 (similar to Pacific seawater) to + 7.9. Many samples show substantial contributions from MORB, but all have ε_(Nd)(0) well below MORB at this locality (ε_(Nd)(0) = + 9.7) in spite of very large enrichments in Nd concentrations. While complete isotopic exchange with water/rock≈ 2 or exchange with anomalous oceanic crust can explain the Sr data, the Nd data require exchange with a reservoir having ε_(Nd)(0) < Pacific seawater. Low-temperature reactions with metalliferous sediments on the ridge flanks may provide such a source. Both Sr and Nd in the Guaymas Basin solution are very different from21°N. ε_(Sr)(0) = + 11.0 and ε_(Nd)(0) = −11.4 and are consistent with the fluid exchanging Sr and Nd with heated sediments having a substantial component of old continental detritus. Some irregularities in the Nd isotopic data reported here indicate that there must be a problem of contamination for some ultra-low-level trace elements during sample collection and processing which requires further attention. Using a simple box model, the estimates for hydrothermal Nd fluxes are compared with fluxes which would be required to maintain the relatively radiogenic value of ε_(Nd)(0) ≈ −3 in the Pacific against the influx of more negative Antarctic waters (ε_(Nd)(0) ≈ −9). It is shown that the hydrothermal flux of Nd from mid-ocean ridges falls far short of that necessary to maintain the isotopic balance. This indicates that weathered material from volcanic terranes (ε_(Nd)(0) ≈ +7) is the most reasonable major source of radiogenic Nd in the Pacific.


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http://dx.doi.org/10.1016/0012-821X(85)90057-3 DOIArticle
http://www.sciencedirect.com/science/article/pii/0012821X85900573PublisherArticle
Additional Information:© 1985 Elsevier Science. Received July 23. 1984. Revised version received November 8. 1984. We would like to thank John Edmond for generously providing us with the samples which were analyzed in this study, and Karen Von Damm for providing us with the Mg concentration data. Discussions with K. Turekian. J. Edmond, K. von Damm and LT. Silver as well as comments on the manuscript by F. Albarède, H. Elderfield and an anonymous reviewer were very helpful in shaping some of the ideas put forth in this paper. We are particularly grateful to K. Turekian for pointing out a 1% error in a constant used in an order of magnitude calculation*. Some reviewers urged us to present a more cautious interpretation in light of possible contamination problems. We hope that a balance between interpretation and license has been achieved. This work was supported in part by grants from the National Science Foundation: OCE 83-08884 and OCE 83-20516 and NASA NGL-05-002-188.
Funders:
Funding AgencyGrant Number
NSFOCE 83-08884
NSFOCE 83-20516
NASANGL-05-002-188
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences3973
Lunatic Asylum Lab453
Record Number:CaltechAUTHORS:20131023-103534320
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20131023-103534320
Official Citation:D.J. Piepgras, G.J. Wasserburg, Strontium and neodymium isotopes in hot springs on the East Pacific Rise and Guaymas Basin, Earth and Planetary Science Letters, Volume 72, Issue 4, March 1985, Pages 341-356, ISSN 0012-821X, http://dx.doi.org/10.1016/0012-821X(85)90057-3. (http://www.sciencedirect.com/science/article/pii/0012821X85900573)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42023
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:23 Oct 2013 18:02
Last Modified:28 Oct 2013 22:35

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